In a conventional filling machine for filling a can with a beverage, such as beer, soft drinks or fruit juice, the cans are fed by a star wheel conveyor to a filling machine and individually positioned on the surface of a rotating turntable beneath a filling head. With the can positioned on the surface, a cylindrical tulip mounted on the head is moved downwardly and sealed against the upper edge of the can. A conventional filling machine also includes a bowl or reservoir which is spaced above the supporting surface and contains the liquid or beverage. A vent tube and fill tube are mounted concentrically within the tulip and the upper ends of both tubes communicate with the bowl.
In a typical filling operation, as disclosed in the U.S. Pat. No. 4,938,261, after the tulip is sealed against the upper edge of the can, pure carbon dioxide from a separate reservoir is purged into the can and air from the can is simultaneously vented to the atmosphere. After the purging, a differential in force applied to opposite ends of the vent tube acts to lower the vent tube into the can, and the fill tube is then lowered to open a valve so that liquid is discharged from the bowl into the can, while gas in the can is discharged through the vent tube to the headspace of the bowl. During the filling cycle, liquid will rise in the can until the liquid level covers the lower end of the vent tube and the liquid will then rise in the vent tube to approximately the level of liquid in the bowl. To release liquid from the vent tube, the vent tube is lifted relative to the tulip to compress the gas in the upper end of the vent tube and force the liquid out of the vent tube and into the can. The gas is then released from the head space in the can to complete the filling operation.
One of the problems that arises in the filling operation is preventing overfill of the can. During the period when the valve is open, the can is filled with liquid until the liquid level closes off the lower end of the vent tube. To prevent overfill under these conditions it has been proposed to employ a narrow gap in the flow path of the liquid flowing to the can. The gap should be large enough to provide an adequate fill time and yet must be correlated with the surface tension of the liquid to prevent flow when the can is filled and the vent tube is closed off. In addition, rotation of the turntable which carries the filling head will tend to decrease the surface tension of the liquid, so that this factor must also be considered in determining the size of the gap.
In U.S. Pat. No. 4,938,261 an annular siphon valve is carried by the fill tube and includes an upturned peripheral edge, which is adapted to engage a seat on the housing. When the valve is open, the space between the peripheral edge of the valve and the valve seat provides a gap, which is intended to prevent overfill. However, it has been found that making the gap narrow enough to create the necessary surface tension and prevent overfill will restrict flow during filling so that the fill time is reduced.
It has also been proposed to incorporate an annular mesh screen in the lower end of the annular flow chamber between the fill tube and the housing, and above the valve. While the use of the screen has been found to provide adequate fill time and will create the desired surface tension to prevent overfill, the screen will not accommodate liquids containing fibrous material or pulp, so that the screen will tend to clog.